AssetManager to parse local JSON file returns NullPointer

Question

I have red many topics and questions about it, but can't find any solution. I am also very new to Android and Java.

What I was trying to do is to parse my local JSON file and to set an adapter to view my Strings in a ListView. Shouldn't be too complicated, but I got this error:

01-10 10:04:38.109 29962-29962/com.thkoeln.stucked E/AndroidRuntime: FATAL EXCEPTION: main
                                                                 Process: com.thkoeln.stucked, PID: 29962
                                                                 java.lang.RuntimeException: Unable to start activity ComponentInfo{com../com...MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.res.AssetManager android.content.Context.getAssets()' on a null object reference
                                                                     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2416)
                                                                     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2476)
                                                                     at android.app.ActivityThread.-wrap11(ActivityThread.java)
                                                                     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1344)
                                                                     at android.os.Handler.dispatchMessage(Handler.java:102)
                                                                     at android.os.Looper.loop(Looper.java:148)
                                                                     at android.app.ActivityThread.main(ActivityThread.java:5417)
                                                                     at java.lang.reflect.Method.invoke(Native Method)
                                                                     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
                                                                     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)
                                                                  Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.res.AssetManager android.content.Context.getAssets()' on a null object reference
                                                                     at android.content.ContextWrapper.getAssets(ContextWrapper.java:81)
                                                                     at com.thkoeln.stucked.JSONParser.loadJSONFromAsset(JSONParser.java:28)
                                                                     at com.thkoeln.stucked.JSONParser.parseJson(JSONParser.java:52)
                                                                     at com.thkoeln.stucked.MainActivity.onCreate(MainActivity.java:24)
                                                                     at android.app.Activity.performCreate(Activity.java:6237)
                                                                     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1107)
                                                                     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2369)
                                                                     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2476) 
                                                                     at android.app.ActivityThread.-wrap11(ActivityThread.java) 
                                                                     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1344) 
                                                                     at android.os.Handler.dispatchMessage(Handler.java:102) 
                                                                     at android.os.Looper.loop(Looper.java:148) 
                                                                     at android.app.ActivityThread.main(ActivityThread.java:5417) 
                                                                     at java.lang.reflect.Method.invoke(Native Method) 
                                                                     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726) 
                                                                     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)

And here is my MainActivity.java :

package com.thkoeln.stucked;
import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.widget.ListView;
import android.widget.ListAdapter;
import android.widget.SimpleAdapter;

public class MainActivity extends AppCompatActivity {

ListView activityList;


protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    activityList = (ListView) findViewById(R.id.activityList);

    String fromArray[] = {"uid", "uname", "ustart"};
    int to[] = {R.id.uid, R.id.uname, R.id.ustart};

    JSONParser jp = new JSONParser();
    jp.parseJson();
    /*ListAdapter adapter = new SimpleAdapter(this, jp.userList, R.layout.user_item, fromArray, to);
    activityList.setAdapter(adapter);*/

    }


}

And this is my JSONParser.java :

package com.thkoeln.stucked;

import android.support.v7.app.AppCompatActivity;
import android.util.Log;
import android.widget.ListView;

import org.json.JSONArray;
import org.json.JSONException;
import org.json.JSONObject;

import java.io.IOException;
import java.io.InputStream;
import java.util.ArrayList;
import java.util.HashMap;


public class JSONParser extends AppCompatActivity {

ArrayList<HashMap<String, String>> userList;

public String loadJSONFromAsset() {
    String json = null;
    try {
        InputStream is = getAssets().open("users.json");

        int size = is.available();

        byte[] buffer = new byte[size];

        is.read(buffer);

        is.close();

        json = new String(buffer, "UTF-8");

    } catch (IOException ex) {
        ex.printStackTrace();
    }
    return json;

}

public JSONObject parseJson() {

    //ArrayList<HashMap<String, String>> userList
    try {

        JSONObject reader = new 
       JSONObject(String.valueOf(loadJSONFromAsset()));

        JSONArray users = reader.getJSONArray("users");
        Log.d("AnzahlUser", String.valueOf(users.length()));

        for (int i = 0; i < users.length(); i++) {
            JSONObject u = users.getJSONObject(i);
            Log.d("Details-->", u.toString());
            String uid = u.getString("uid");
            String uname = u.getString("uname");
            String ustart = u.getString("ustart");
            //String uend = u.getString("u.end");

            HashMap<String, String> user = new HashMap<>();

            user.put("uid", uid);
            user.put("uname", uname);
            user.put("ustart", ustart);
            //user.put("u.end", uend);

            userList.add(user);




        }
    } catch (final JSONException e) {
        e.printStackTrace();
    }

    return null;
    }
}

I would be so thankful, if someone could help me with these ...

Obviosuly you should not use `operator new` with class derived from `Activity` ... obvious choice is to make `loadJSONFromAsset/parseJson` static method with `Context` as parameter — Selvin, Jan 10 '18 at 10:20
Your Json Parser object has no reference to a context or activity. I think you forgot to do that — Niza Siwale, Jan 10 '18 at 10:21

score -1 · Accepted Answer · answered Jan 10 '18 at 10:23

You should not extend AppCompatActivity for context. You should pass context to the JSONParser class to get Asset.

You can only get context for an activity if it is attached to the android life cycle.

Your JSONParser should be like this.

public class JSONParser{
private Context context;

public JSONParser(Context context){
   this.context =  context;
}

ArrayList<HashMap<String, String>> userList;

public String loadJSONFromAsset() {
    String json = null;
    try {
        InputStream is = context.getAssets().open("users.json");

        int size = is.available();

        byte[] buffer = new byte[size];

        is.read(buffer);

        is.close();

        json = new String(buffer, "UTF-8");

    } catch (IOException ex) {
        ex.printStackTrace();
    }
    return json;

}

public JSONObject parseJson() {

    //ArrayList<HashMap<String, String>> userList
    try {

        JSONObject reader = new 
       JSONObject(String.valueOf(loadJSONFromAsset()));

        JSONArray users = reader.getJSONArray("users");
        Log.d("AnzahlUser", String.valueOf(users.length()));

        for (int i = 0; i < users.length(); i++) {
            JSONObject u = users.getJSONObject(i);
            Log.d("Details-->", u.toString());
            String uid = u.getString("uid");
            String uname = u.getString("uname");
            String ustart = u.getString("ustart");
            //String uend = u.getString("u.end");

            HashMap<String, String> user = new HashMap<>();

            user.put("uid", uid);
            user.put("uname", uname);
            user.put("ustart", ustart);
            //user.put("u.end", uend);

            userList.add(user);




        }
    } catch (final JSONException e) {
        e.printStackTrace();
    }

    return null;
    }
}

Your MainActivity should be like this.

public class MainActivity extends AppCompatActivity {

ListView activityList;


protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    activityList = (ListView) findViewById(R.id.activityList);

    String fromArray[] = {"uid", "uname", "ustart"};
    int to[] = {R.id.uid, R.id.uname, R.id.ustart};

    JSONParser jp = new JSONParser(this);
    jp.parseJson();
    /*ListAdapter adapter = new SimpleAdapter(this, jp.userList, R.layout.user_item, fromArray, to);
    activityList.setAdapter(adapter);*/

    }


}

That helped, and I am sorry, very simple but I am still learning. So big thank you !! — jrappen1, Jan 10 '18 at 10:32

AssetManager to parse local JSON file returns NullPointer

1 Answers1